1. Field of the Invention
The present invention relates to a transmitter of a wireless appliance employing a digital modulation system.
2. Description of the Prior Art
A transmitter employing a digital modulation system has been developed and disclosed in the U.S. patent application Ser. No. 07/777,012. In this appliance, the distortion of a power amplifier is compensated by extracting a part of the transmission signal from the power amplifier, comparing the transmission envelope signal in which the signal is detected of the envelope and a distortion-free envelope signal in an error detector to generate an error signal, and controlling the gain of a gain control amplifier by using this error signal as the gain control signal. This appliance, however, has many aspects to be improved upon. First, generally, a variable gain control amplifier is controlled by a positive DC voltage to some degree. In this appliance, an output voltage of a error amplifier is fixed by the gain of the error amplifier, and a voltage difference between the transmission envelope signal and the distortion-free reference envelope signal. This output voltage of -the error amplifier is directly used as a control voltage of the gain control amplifier.
Here, since the gain of the error amplifier should be relatively small in consideration of the stability of the loop circuit, the voltage difference between the transmission envelope signal and the distortion-free reference envelope signal cannot be made too small. A voltage of the variable gain control amplifier (Vg) is obtained as: EQU Vg=(Vcont-vdeta).times.G
where G.times., Vcont and Vdeta are a gain of the error amplifier, a voltage of the distortion-free reference envelope signal, and a voltage of the transmission envelope signal, respectively. This equation can be transformed as EQU Vdeta=Vcont-vg/G,
The closer Vdeta is to Vcont, the higher the precision of linearity compensation is. This appliance, however, does not provide a superior precision of lineality because of the restriction of the gain of the error amplifier (G).
Furthermore, when an attempt is made to expand the output power range of the transmission output, the input power range of the envelope detection circuit for detecting the transmission monitor circuit monitor output becomes wide, possibly exceeding the dynamic range of the transmission monitor circuit monitor output power corresponding to the detection voltage in a certain determined range.